Method of electrophotography with development on the substrate surface of the photoconductive article

ABSTRACT

APPARATUS AND METHOD FOR ELELCTROSTATICALLY REPRODUCING IMAGES UPON RECORDING MEDIA IN WHICH AN INSULATING SHEETLIKE MEMEBER IS COATED ON ONE SIDE WITH A PHOTOCONDUCTIVE LAYER. THE PHOTOCONDUCTIVE LAYER IS CHARGED SO AS TO PRODUCE A LATENT IMAGE ON THE UNCOATED FACE OF THE INSULATING SHEET-LIKE MEMBER. TONER IS APPLIED TO THE LATENT IMAGE TO PRODUCE A VISIBLE IMAGE. VARIOUS MEANS ARE DISCLOSED FOR APPLYING THE TONER, TRANSFERRING THE LATENT IMAGE TO OTHER CARRIER SHEETS AND FOR SELECTIVELY CHARGING THE PHOTOCONDUCTIVE SHEET TO PRODUCE THE LATENT IMAGE.

July 20, 1971 A. L. KAUFMAN 3,594,161

METHOD OF ELECTROPHOTOGRAPHY WITH DEVELOPMENT ON THE SUBSTRATE SURFACE 0F THE PHOTOCNDUCTIVE ARTICLE Filed July 24. 1968 4 Shaets-Sheet l E F/G 5 i@ 79 WW/WM ////i tf f5 @l W fm ,M w i L7/4 /7/ July 20, 1971 A. KAUFMAN 3,594,151

METHOD OF ELECTROPHOTOGRAPHY WITH DEVELOPMENT 0N THE susTRATE SURFACE oF THE PHoTocoNDucTIvE ARTICLE Filed July 24, 1968 4 Sheets-Sheet 2 F/G. a 2f F/G- 9 July 20, 1971 A. L. KAUFMAN 3.594.161

METHOD OF ELECTROPHOTOGRAPHY WITH DEVELOPMENT 0N THE SUBSTRATE SURFACE OF THE PHOTOCONDUCTIVE ARTICLE Filed July 24. 196e 4 sheets-sheet :s

July 20, 1971 A. L. KAUFMAN 3,594,161

MEHOD oF ELEcTRoPHoToGRAPHY WITH DEVELOPMENT QN TEE suBsTEATE SURFACE oF THE PHoToczoNDUCTIVEl ARTICLE Filed July 24. 1968 4 Sheets-Sheet 4 F/G. /a

United States Patent O 3,594,161 METHOD F ELECTROPHOTOGRAPHY WITH DE- VELOPMENT ON THE SUBSTRATE SURFACE OF THE PHOTOCONDUCTIVE ARTICLE Arthur L. Kaufman, 26 High Point Road, Westport, Conn. 06880 Continuation-impart of abandoned application Ser. No. 434,078, Feb. 10, 1965. This application July 24, 1968, Ser. No. 747,163

Int. Cl. G03g 13/22 U.S. Cl. 96-1 13 Claims ABSTRACT OF THE DISCLOSURE Apparatus and method for electrostatically reproducing images upon recording media in which an insulating sheetlike member is coated on one side with a photoconductive layer. The photoconductive layer is charged so as to produce a latent image on the uncoated face of the insulating sheet-like member. Toner is applied to the latent image to produce a visible image. Various means are disclosed for applying the toner, transferring the latent image to other carrier sheets and for selectively charging the photoconductive sheet to produce the latent image.

BACKGROUND OF THE DISCLOSURE This invention relates to the electrostatic reproduction of graphic information and is a continuation-in-part of an application filed' Feb. 10, 1965, Ser. N0. 434,078, now abandoned entitled: Electrostatic Reproducing Apparatus and Method, Arthur L. Kaufman, applicant.

In electrostatic reproduction generally, a latent image is rst created in the form of a pattern of electrostatic charges on a surface, the charge pattern corresponding to the indicia to be reproduced. The latent electrostatic image is converted to a visible image by the application of a toner, which adheres selectively either to the charged or the uncharged portions of the surface. The resulting visible image is either transferred to another surface and there fixed or rendered permanent, or it may be made permanent on the surface on which it is originally created, depending on the process involved.

The creation of the latent image is accomplished by the use of a surface coated with a photoconductor, that is, a material having the property of changing its electrical resistance upon exposure to radiation. The photoconductor may be an inorganic material such as selenium, zinc oxide, phosphors and various other inorganic materials known to the art. It may also be an organic material exhibiting photoconductive properties, such as anthracene. In either case, it may be sensitized to Wave lengths other than those to which it normally responds, by the addition of absorptive dyes and the like, as is also well known in the art. Photoconductive compositions may be prepared in this way, havin-g substantially any desired absorption band for actinic radiation ranging from X-rays through the ultra-violet, visable and into the infra-red portions of the spectrum.

To prepare a photoconductive surface useful in electrostatic reproduction, the selected photoconductor 1s applied to a backing substrate in any convenient manner. Among the methods commonly used to apply the photoconductor are vacuum deposition, spraying, rolling, dipping, etc. The photoconductor is usually applied as a lm of a thickness in the range between one or two microns and a few thousandths of an inch.

The substrate material is usually metal, paper, plastic or glass with a conductive coating, such as a stannic oxide coating. In any case, the substrate material is either inherently conductive or is designed and if necessary modified so as to be relatively conductive as compared to the photoconductive layer. This conductivity is required in order to aid in the charging of the photoconductive layer, and the subsequent steps necessary to create the latent charge image and convert it to a visible image.

The actual production of the image is accomplished by first sensitizing the photoconductor, by charging it in the dark with a uniform overall electrostatic charge. Charging is done by subjecting it to a charging field which may be a single or double corona discharge, one or more charged rollers or an electrostatic field between suitable electrodes.

The charged, or sensitized, surface is then exposed' to a beam of light image bearing the indicia to be reproduced. I-n proportion to the amount of light striking each portion of the photoconductive surface, it becomes conductive, and the charge on the surface of the photoconductive matten'al acts as though conducted through the photoconductive layer to neutralize the charge at the interface. The substrate may appropriately be grounded, be an insulator or, in the case of a massive substrate such as a metal roll, may serve as its own ground. In the areas not thus discharged by the action of the light, the photoconductive surface retains the charge originally imparted to it. The condition of the photoconductive surface after exposure, therefore, is that it has a pattern of electrical charges corresponding to the variations in intensity of the light image. This visible pattern of charges is known as the latent charge image.

It is not in all cases necessary to charge the photoconductive layer in order to sensitize it. In some electrostatic reproduction processes, the photoconductive layer is simply allowed to become dark-adapted, by keeping it in the dark. In the dark-adapted condition, the photoconductive layer is uniformly resistive over its entire surface. The dark-adapted photoconductive layer is then exposed to a light image. In those areas impinged upon by the light, the layer becomes electrically conductive, While the remainder of the layer remains in the resistive state. After exposure to the light ima-ge, the photoconductive material does not return instantly to its darkadapted resisti-ve state, but remains conductive for a finite interval, the length of which depends on the particular photoconductive composition. The condition of the photoconductive surface just after light exposure, then, is that its conductivity varies from point to point, depending on how much light has struck each point. This type of latent image is referred to as a latent conductivity 1mage.

Still another method of creating a latent image does not necessarily involve exposure to light, nor does it require the use of a photoconductive layer. Instead, the latent image is created on an assembly consisting of a relatively insulating surface backed up by a relatively conducting layer. The agency for creating the latent image is, instead of light, a point discharge from a metal pin electrode. The discharge directly creates a latent charge image on the surface, similar to the latent charge image produced by the action of light on a charged photoconductive layer.

Toning the latent image The next step in the operation is to convert the latent image (whether a latent charge image or a latent conductivity image) to a visible image. This step is known as toning, and the materials used for the purpose are known as toners. The toner must be a material which can be made to deposit selectively on the exposed, or alternatively on the unexposed areas of the surface bearing the latent image. In the case of a latent charge image, the usual approach is to use a toner in the form of particles which can be charged. If positively charged they are know as positive toners, and will be attracted selectively to the negatively-charged portions of the latent image, i.e. those portions that have not been discharged by the action of light. They therefore create an image which is light where the original light image was light, and dark where it was dark. They therefore produce a positive copy from a positive original and negative copy from a negative original.

By the same token, a toner in the form of negativelycharged particles is known as a negative toner. It produces negative copy from a positive original and positive copy from a negative original.

Several methods are commonly used for imparting the necessary charge to the particles of toner. The toner may be used in a conductive or non-conductive triboelectric charging carrier, in either liquid, gelatinous or solid form. Alternatively, toners may be charged in an aerosol, for example, by passing the aerosol particles between charging electrodes, so that the particles will be attracted to the appropriate parts of the latent image. In still another variation, triboelectrically neutral, uncharged, relatively conductive particles, without a carrier, can be brought near to the charge image and made to adhere to the photoconductive surface by electrostatic induction.

Various methods are known to the art for enhancing the quality of the visible image produced in the toning operation. lCounter electrodes, with or without additional circuit voltage, may be placed above the photoconductive surface and/or against the relatively conductive substrate support to achieve various types of image enhancement such as cleaning of background, reduction or increase of image density, increased and more uniform fill-in of large solid areas, etc.

The electrostatic reproduction systems heretofore proposed have been highly successful and possess distinct advantages in many respects over previously available methods. Nevertheless, they have suffered from a number of disadvantages.

A number of prior art disadvantages result from the fact that in many of the electrostatic reproduction methods, the photoconductive layer is on the surface of the sheet which is to bear the nal, permanent image. For example, when the toning is to be done on top of the photoconductive layer, the photoconductor layer determines, to some extent, the characteristics of the surface to be toned, and the characteristics imposed by the nature of the photoconductive layer are not necessarily those most conductive to reception and retention of the toner.

Also, the desired actinic response may dictate the use of photoconductive materials having pronounced and sometimes undesirable colors such as those of yellow cadmium suliide, or black lead sulfide, or muddy grayish tones such as are frequently exhibited by heavily dyesensitized zinc oxide photoconductive materials. These colored materials are commonly undesirable as the background color of the toned image.

When toning on top of the photosensitive material, from an opaque original, it is necessary to provide an optical system which will reverse the image from left to right, because a reflex contact print would be a mirror-image reversal of the original. Such an optical system may comprise mirrors or prisms to reflect the original an odd number of times enroute to the photoconductive layer. The necessity of providing such an optical system increases the size and cost of the apparatus.

The record materials presently used for this type of reproduction, `consisting of a sheet substrate surface-coated with a photoconductive material, suffer from variations in ambient humidity. Unless special and expensive precautions are taken to control the atmosphere, the ambient humidity may vary from l or less to 85 or more percent relative humidity. These variations seriously affect the conductivity of the substrate. Changes in substrate conductivity, in turn, affect charge retention and image quality. If the substrate is too conductive, the charge tends to leak away even where not exposed to light; if too resistive, it

may prevent a good uniform charge on the photoconductive layer during charging.

Many of the above disadvantages, of course, are evaded by those electrostatic reproduction systems which employ the expedient of making the initial exposure on a photoconductive surface, toning the image thereon, and transferring the toner image from the photoconductive surface to another surface such as a sheet of paper, before fusing the particles, or otherwise rendering permanent the toner image. This technique is not a complete answer to the problems of electrostatic reproduction, however, as there remain numerous diiiculties common to this method and the methods in which the toner is permanently applied directly over the photoconductive layer.

All existing systems of electrostatic reproduction, for example, require the handling of toner materials in bulk form, either as solutions, gels, loose powders, aerosols or the like. Moreover, all existing methods require either that some means be provided for charging the toner particles, or else that means be provided for bringing the toner particles close enough to the photoconductive layer so that they can be attracted by induced charges. To satisy these requirements, the user of the system must early stocks of toner, replenisher, carrier, etc., and also provide himself with suitable applicator equipment for charging the particles, directing them toward the photoconductive layer, etc.

Another difficulty associated with the use of toners in bulk form resides in the requirements of the dispersant in which such toners must frequently be carried. In order to make a dry, usable image in a reasonably short time, it is necessary that such a dispersant be volatile enough to evaporate rather quickly. Such dispersants, however, have a short pot life because they tend to evaporate while waiting to be used.

An object of the present invention, is to provide methods, apparatus and recording media for electrostatic reproduction which will overcome the deficiencies of prior art devices.

Another object of the present invention is to provide electrostatic reproduction methods and materials which avoid the necessity of producing a permanent toned image on top of a photoconductive layer.

Still another object of the present invention is to provide an electrostatic reproduction system whereby a rightreading copy can be produced by reflex contact printing from an opaque original.

Another object of the present invention is to provide an electrostatic reproduction system of reduced dependency upon ambient humidity conditions.

Another object of the present invention is to provide an electrostatic reproduction system which does not require the use of toner materials in bulk form.

A further object of the present invention is to provide a system as aforesaid, which eliminates the use of shortpot-life toner compositions.

A still further object of the present invention is to provide an electrostatic reproduction method capable of the reproduction of half-tones and as subjects requiring rapid processing.

A feature of the present invention is the application of toners to the side of the substrate opposite the side which carries the photoconductive layer.

Another feature of the present invention is the use of a substrate sheet which is relatively insulating as compared to conventional conductive substrates.

Other features of the present invention found singly or in combination in various embodiments of the invention, include the following:

(l) Use of a substrate having a resistivity lying between certain practical limits more fully set forth hereinafter.

(2) Use of transparent photoconductive layers, especially on transparent substrates.

(3) Application of toner to one side only of the substrate sheet.

(4) Use of a temporary cover lm on the photoconductive side of the substrate to permit toning by immersion while preventing the toner from reaching the photoconductive side of the sheet.

(5) Use of a thin colored, opaque or translucent layer behind the substrate or between the substrate and the photoconductive layer.

(6) Use of a black, white or colored substrate layer.

(7) Use of encapsulated toners applied to the side of the substrate sheet opposite the photoconductive layer.

(8) Use of pressure rolls to break toner capsules after exposure of the sheet, permitting the toner to tone the latent image. y

(9) Use of persistently polarized layers, in place of conventional photoconductive materials.

(10) Increasing the effective speed of record materials by increasing the sensitivity of toner to small differences in charge intensity in the latent image.

(1l) The use of a charged pressure roller to provide blocking or aiding voltages for the enhancement of image quality.

(12) Use of a tacky release sheet to remove from the substrate excess toner not used in image formation.

(13) Use of a tacky-backed substrate to hold toner deposits in image area.

(14) Use of a tacky release sheet to form a permanent image.

(15) Use of a smooth-backed substrate for forming a temporary, transferable image.

(16) Production of multiple copies from a single charge and exposure of the photoconductive layer, by repeatedly toning the rear side of the substrate, lifting the toned image 01T onto a release sheet, and retoning the back of the substrate.

(17) Simultaneous exposure and development of a sheet of electrostatic record material, using a transparent, internally lighted pressure roll to press a transparent original against the photoconductive layer While releasing toner in contact with the opposite side of the substrate.

(18) Production of visible changing images, such as uoroscope displays, for example, by retaining liquid or otherwise mobile toner in proximity to one side of a sheet which is coated on the other side with photoconductive material exposed to radiation forming the changing image.

(19) The use of toner in the form of a sheet coated with encapsulated toner, separate from the substrate which carries the photoconductive material.

(20) Application of encapsulated toner as a coating on top of the photoconductive layer, and exposing the photoconductive layer through the toner layer.

(21) Deposition of toner capsules on a supporting surface through a half-tone screen or equivalent, to break up solid areas and provide for reproduction of continuous tone copy.

(22) Production of a series of superposed images of different colors (e.g. color separation negatives or 5-color map images) on the side of a substrate opposite the side carrying the photoconductive layer, thereby avoiding desensitization of the photoconductive layer by the rst toned image.

(23) Use of a record sheet coated with encapsulated toner, to be placed against the back of a substrate sheet with the toner capsules facing away from the substrate sheet.

(24) Preparation of multiple copies simultaneously by stacking thin, relatively insulating record sheets, each with a coating of encapsulated toner, behind a single substrate carrying a charged, exposed photoconductive layer.

The invention consists of the construction, combination and arrangement of parts, as herein illustrated, described and claimed.

In the accompanying drawings, forming a part hereof, there are illustrated several embodiments of the invention, in which drawings similar reference characters designate corresponding parts.

SUMMARY OF THE INVENTION One particularly desirable embodiment of this invention contemplates the use of reproducing indicia comprising a reco-rd medium having a relatively insulative substrate sheet and a coating of photoconductive material on one face of said sheet. The said record medium is exposed to an active radiation image of the indicia t0 be reproduced, thereby crea-ting on said record medium a latent image corresponding to said indicia. A visible image of said indicia is then developed by applying an electrostatically sensitive toner to the face of said substrate sheet opposite the face bearing said photoconductive material.

Conventional methods ordinarily employ a highly conductive substrate, such as a metal platen or roll, and it has also been reported in the literature that backing sheets will act as good conducting substrates with resistivities as high as 1X1()6 to l l08 ohm-cm. It has been discovered that when an insulative charge storage layer, whether of a photoconductive or light insensitive insulative material, is placed on a relatively insnlative substrate that lines of electrostatic ux extend through to the rear of the relatively insulative substrate with sufcient strength to enable the toning of an image on the rear thereof. This toned visible image being an accurate reection of the latent charge image stored in the top insulative charge storage layer. The term relatively insulative substrate as used herein, is intended to mean a supporting sheet-like member, having a resistivity not less than 1X 101 ohm-cm. and not more than two orders of magnitude greater than that of the insulative charge storage layer.

Insulative charge storage layers, such as light sensitive photoconductors when dark adapted, e.g. zinc oxide in a resinous binder, or light insensitive layers, e.g. polyethylene, have resistivity values that are typically l l013 ohm-cms. or greater. Examples of such materials and their resistivities are:

zinc oxide in a resinous binder-1X1013-1X 1018 ohmcms.

polyethylene-1X 1016-1 X 1011 ohm-cms.,

Mylar-1 101'1 ohm-cms., etc.

Relatively insulative substrate layers that permit the electrostatic lines of ilux from the top layers latent charge image to extend through to the rear of the substrate without acting to short out the llux lines and hence with sufficient strength to attract toner may have resistivities as low as 1 10111 ohm-cms. However, on the high side, the resistivity range is preferably no more than two orders of magnitude greater than that of the insulative `charge storage layer. For example, if the insulative charge storage layer is dark adapted zinc oxide in a particular resinous binder that has a resistivity of 1 1015 ohm-cms. then the relatively insulative substrate layer may go down to 1 1010 ohm-cms. but no higher than 1X 1017 ohm-cms. Similarly, if the insulative charge storage layer is a coating of anthracene with a dark adapted resistivity of 1 1013 ohm-cms. then the relatively insulatively insulative substrate may again go down to 1 101o ohm-cms. but should go no higher than 1 1015 ohm-cms. A third example might be a light insensitive insulative charge storage layer of polyethylene having a resistivity of 1 1017 ohm-cms. on which the latent charged image was placed by means of electrostatic discharge pins, etc. The relatively insulative base layer may again go down to 1 1010 ohm-cms. but in this case should go no higher than 1 l019 ohm-cms. Typical relatively insulative substrates are, cellulose acetate films, and dried papers.

Experiments to-date with a variety of zinc oxide and resinous binder combinations used as the insulative charge storage layer show that the best rear of sheet toning images are obtained when the relatively insulative substrates resistivity lies between 1 1012 ohm-cms. and 1 1014 ohm-cms. Experiments have further shown that below 1 1012 ohm-cms. the image edges begin to lose sharpness and that above l 1011 ohm-cms. some spurious background begins to form. If values of base sheet resistivity extend more than two orders of magnitude above that of the zinc oxide and resinous Ibinder insulative charge storage layer, no image is formed and there is only a large amount of spurious background present.

The actinic radiation to which the photoconductive material is exposed will most commonly be visible light, but may also be infra-red, ultra-violet or X-radiation, as indicated above. In some cases, the actinic radiation is secondary emission produced by a stream of electrons impinging on a phosphor surface, as produced in a cathode-ray tube. In some embodiments of the invention, moreover, the photoconductive layer may be dispensed with, and the latent image produced by other means directly on the front face of the insulating sheet, for example, by the use of a charged metal pin or array of pins, a moving charged stylus, r the like.

The latent image may be either a latent charge image or a latent conductivity image.

The electrostatically sensitive toner may be any of the conventional materials as presently employed in commercial practice. It may also be a liquid toner in encapsulated form, i.e. in the form of minute droplets, each coated with a thin shell of solid material such as gelatin, paraffin or the like, by methods well known to the art for encapsulating inks and liquids in general.

If desired, the toner may be encapsulated in the form of a single frangible pod, with provision of pressure rollers or equivalent means for breaking the pod and spreading the contents over the image surface.

A significant departure of the present invention from conventional electrostatic reproduction systems lies in the fact that the toner is not applied to the surface of the photoconductive material, but rather to the back of the substrate; that is, the face of the substrate remote from the photoconductive material.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic representation greatly enlarged of a cross-section through a substrate sheet and photoconductive coating according to the invention, showing a portion of the force field associated with a latent charge image. y

FIG. 2 is a diagrammatic representation similar to FIG. 1, showing a conventional conductive substrate with a latent charge image.

FIG. 3 is a diagrammatic representation of a crosssection through a substrate and photoconductive coating according to the invention, showing certain additional features of a force field associated with a latent charge image.

FIG. 4 is a fragmentary cross-section, on a greatly enlarged scale, of a composite record medium according to another embodiment of the invention.

FIGS. 5 and 6 are cross-sectional views similar to FIG. 4, of two additional embodiments of the invention.

FIG. 7 is a cross-sectional fragmentary View of another embodiment of the invention in the form of a sheet coated with encapsulated toner particles.

FIG. 8 is a somewhat diagrammatic cross-sectional view of an apparatus useful with the sheet of FIG. 7.

FIG. 9 is a view similar to FIG. 8, showing a modified form of apparatus.

FIG. l0 is a View similar to FIG. 9, showing a modiiication wherein the toner capsules are supplied from a release sheet.

FIG. 1l is a diagrammatice cross-sectional view of another modiiied form of apparatus, useful for simultaneously exposing and developing the image.

FIG. l2 is a cross-sectional representation of another 8 apparatus according to the invention, namely an image wavelength converter cell. cording to still another embodiment of the invention, hav- FIG. 13 is a fragmentary cross-section of a sheet according to another embodiment of the invention, designed to be exposed through a transparent substrate.

FIG. 14 is a fragmentary cross-section of a sheet acing an encapsulated toner in the form of a pattern of spaced dots.

FIG. 15 is an isometric view of an apparatus according to another embodiment of the invention, for generating indicia from electrostatic discharges from an array of metal pins or the like.

FIG. 16 is a diagrammatic cross-sectional view of microlm reader-printer according to the invention.

FIG. 17 is a cross-sectional view of a cathode-ray tube print-out system according to the invention.

FIG. 18 is a diagrammatic cross-section of another embodiment of the invention in which the sheet bearing the latent image is not itself toned, but directs the deposit of toner on a separate sheet.

DESCRIPTION OF THE PREFERRED EMBODI- MENTS-THEORETICAL DISCUSSION Referring particularly to FIG. 1, there is shown a relatively insulating substrate 10, according to the invention, and a photoconductive layer 11 thereon. The photoconductive layer 11, is represented as bearing a charge, which may be a portion of a latent charge image, symbolized by the minus signs on the outer surface of the photoconductive layer and the plus signs on the surface in contact with the substrate 11. The lines of flux surrounding the charge are represented by curved lines 13, and the convention is employed of representing the directionality of the lines of ux by arrows indicating the path from the site of negative charge to the site of the positive charge.

Comparing FIG. 1 with FIG. 2, it can be seen that, whereas in FIG. l the lines of ux 13 (and hence the force iield) extend outwardly behind the relatively insulative substrate 11, in FIG. 2 the lines of iiux 13a and the force field are shorted out through the relatively conductive substrate 12.

Since the air above the photoconductive layer is also an insulator, the electrostatic lines of ux also extend up into it, perhaps somewhat further than they extend to the rear, as the air is usually a better insulator than the substrate.

The character of the field around the latent charge image is further analyzed in FIG. 3, showing a single line of flux 13, forming a closed curve from the negatively charged front face of a charge-bearing portion of the image surface, through the air above the photoconductive layer 11, through a non-charged portion of the photoconductive layer 11, and the substrate 10, through the air behind the substrate 10, and then through the substrate to the positively charged rear face of the photoconductive layer 11.

The electrostatic line of flux 13 is resolved into horizontal and verticle components as indicated by dotted arrows. 'Ihe vertical vectors represent that component of the eld which is effective to attract a charged toner particle to the surface, or repel it therefrom. For example, if a positively charged particle is above the negativelycharged image area, it will be attracted by the verticle field vector emanating from the photoconductor surface. A negatively charged particle would be repelled by such a vertical field vector. By the same reasoning,a negatively-charged toner particle will be attracted to the top surface of the photoconductor where the electrostatic line of flux reenters the photoconductor, whereas a positivelycharged particle would be repelled by the vertical iield vector acting at this point. Thus, a positive toner will produce a positive image, i.e. one which is black where the original is black, and white where the original is white. Negative toners produce negative images, i.e. reversed as to the light and dark values from those of the original.

The above is reversed if the charge on the photoconductor is positive, instead of negative. In such a case, a positive toner would produce a negative image, and a negative toner a positive image.

All of the foregoing discussion relates to conventional electrostatic reproduction processes, i.e. those in which the toner is applied on the front, i.e. on the exposed face of the photoconductive layer.

In accordance with the present invention, the toner is applied to the opposite face of the substrate from the face of the substrate bearing the photoconductive layer, i.e. the face commonly referred to as the back of the substrate. It is so referred to herein, although in actual operation of the process, it may be more natural to consider this face, on which the toned image appears, as the front, and the photoconductor face as the back of the sheet.

Referring again to PIG. 3, it will be seen that toner particles in back of the sheet are differently affected by the rfield than as those in front. In the charged areas, for example, the vertical component of the field behind the sheet is similar in magnitude to that in front, but opposite in direction. The result is that, in back of the substrate, the charged areas attract negative toner particles and repel positive ones, which is precisely the opposite of the eifect observed on the front. By the same token, the neighboring uncharged areas attract positive toner particles and'repel negative ones. When the toner is applied to the rear, therefore, a positive toner produces a negative image, and a negative toner a positive image. These effects, like those obtaining on the front, are reversed in the case where the charge on the photoconductor is positive, instead of the usual negative charge.

The horizontal components of the field tend to impart a translational motion to the particles, moving them away from areas to which they are not attracted and toward areas to which they are attracted.

From the above discussion it will readily be apparent that wherrusing conventional substrates,effective toning on the back of the substrate is not possible, because the ilux lines are shorted out through the substrate (see FIG. 2) and there is no effective field behind the substrate.

In order to illustrate more fully the nature of the invention and the manner of practicing the same, various specific embodiments will now be described:

lEXAMPLE I A common zinc oxide photoconductor such as Florence Green Seal 8, manufactured by the New Jersey Zinc Co., is mixed in a ball mill for several hours with a silicon resin such as SR-82, manufactured by the General Electric Co., in a ratio of four-to-one by weight of dried solids. The mixture is then coated onto a 0.003 inch thick sheet of clear acetate having a matte surface on the face opposite the coating. Coating is done with a number 30 Wire coating rod, to a thickness of 0.0002 to 0.0005 inch. No dye is added, and the actinic response is that of the zinc oxide alone, i.e. approximately 3,500 to 3,900- ang- Stroms.

The volume resistivity of the acetate sheet is of the order of 1 10l5 ohm-cm., and that of the coating of the photoconductor and the binder is between 1 l013 and 1X l018 ohm-cm., in the dark-adapted state.

After coating, the sheet is allowed to dry and darkadapted by storing it in the dark.

The dried and dark-adapted sheet is charged in the dark by placing it on an aluminum ground plate with the coated side facing up, and passing a negative 6,000 volt, single corona charger over it several times. A double corona discharge may be used in place of the single corona discharge, with equally good results.

The sheet is then exposed by placing it on top of a transparent film original of line and letter copy, with the zinc oxide face upon the film original. The sheet is then exposed to light from a conventional light source, diffused through an opal glass plate. This is done by placing the lm original and the coated substrate sheet on top of the opal glass plate under sufficient pressure to assure close uniform contact. This is necessary in order to avoid undercutting the image. The light source may consist of 15 7-watt, 115 volt diffuse white incandescent bulbs, uniformly spaced over a 9 x 14 inch area, two inches below the opal glass. The exposure is continued for ten seconds.

After exposure, the zinc oxide coated sheet is separated from the original and placed rear face up in a glass tray of liquid positive toner and dielectric dispersant for several seconds as commonly practiced in the art. A toner of this type is available, for example, from the SCM Corporation. Upon removal from the toner liquid, the iilm is found to have a positive image on top of the photoconductive layer and a negative image on the back of the substrate acetate sheet, thus conforming in all respects to the above description.

EXAMPLE II The procedure of Example I is repeated, except that after charging, the acetate sheet was placed against the opal glass with the zinc oxide layer facing up (away from the glass) and in contact with an opaque original. The oxide layer was exposed for ten seconds, and a commercial negative red-pigmented dry toner and iron carrier, mixed in a ratio of thirty parts of iron to one part of pigment by weight, Was dusted over the back of the acetate substrate with a magnetic brush. A positive right-reading image of the opaque original immediately becomes visible.

The sheet is heat fused to render the image permanent. The solid areas are dense and the background clean, the resolution is good.

EXAMPLE III Acetate sheets 0.003 inch thick are coated with a double layer of coating, using as a first coat the same undyed zinc oxide as used in Examples 1 and 2, and as a second thinner coat a similar mixture but with a slight addition of rose bengal dye. The purpose of the dye is to extend the wavelength response into the visible part of the spectrum. This coating increases the effective actinic speed of the photoconductive layer many times, and reflux exposed images under conditions otherwise similar to those of Example 2, can be obtained using exposures of 0.1 to 0.2 second, instead of ten seconds.

EXAMPLE IV An acetate sheet of the type used in the preceding examples is coated with a solution of anthracene in a suitable carrier, and the volatiles are allowed to evaporate, leaving on the acetate substrate a coating of anthracene approximately 0.003 inch thick. The coating is transparent and its presence is barely discernable on the sheet. The sheet is reflex-exposed to an opaque original, with the anthracene layer in contact with the face of the original. After exposure, the back (uncoated) face of the acetate sheet is dusted with a negative dry toner, and a visible positive, right-reading image of the original appears thereon. The image is rendered permanent by heat or vapor-fusing the toner. The -nal product is a positive transparency corresponding to the opaque original.

Instead of anthracene, other transparent photoconductive materials may be used, for example those described by Kalle, or in U.S. Pat. No. 3,140,946.

From the foregoing examples, it will be apparent to those skilled in the art that the present invention offers a number of advantages over conventional methods of electrostatic reproduction.

With no change in the optics or mechanical arrangement of the system, it is possible to produce opaque positive, opaque negative, transparent positive or transparent negative reproductions, merely by an appropriate selection of substrate and toner. It is also possible, and it is believed to be a unique advantage of the present invention, to produce a positive image on one side and a negative on the other side of the same opaque sheet.

Various other alternatives and variations may be practiced in the same general way as described in the above examples. For example, a thin colored layer, a layer which is opaque or translucent, may be added to the base sheet for various purposes, for example, to improve appearance or legibility of the copy, or to distinguish different classes of copy by color-coding the sheets on which they are reproduced. Such an added sheet may be interposed between the photoconductive layer and the substrate or may be on the back of the substrate. :Such modied record sheets are diagrammatically illustrated in FIGS. 4-6.

In FIG. 4, a thin, colored relatively insulating layer 1-4, which may be either opaque or translucent, is interposed between the relatively insulating substrate and the photoconductive layer 11. Such a sheet may be formed by rst laminating sheets 10 and 14 together, and subsequently coating the composite sheet on the face represented by layer 14 with a suitable photoconductive material. Particles 15 of toner are also depicted on the back surface of relatively insulating substrate 10. A light ray 16, incident upon an untoned portion of the substrate 10 is transmitted through the sheet as indicated by dotted arrow 117, emerging as a ray having the color of thin layer 14, assuming the thin layer and the substrate are both transparent. If either is opaque, the light will be reiiected as indicated by arrow 18. If the assembly is translucent, the incident ray will be partially transmitted and partially reflected, both rays taking on the color of layer 14.

FIG. 5 shows another embodiment, similar to that of FIG. 4, except that the additional layer 14a is placed on the back of the relatively insulating substrate 10, instead of being interposed between the substrate and the photoconductive layer. As illustrated in FIG. 5, the sheet is designed for viewing by transmitted light, and layer 14a is either transparent or translucent, as are substrate 10a and photoconducti-ve layer 11a. Incident light rays are transmitted through the untoned portions of the sheet as indicated by arrows 17 and absorbed at the toned portions as indicated by arrows 19.

In the embodiment according to FIG. 6, the image is intended to be viewed by reflected light, and the relative positions of the layers is the same as in FIG. 5. Thin layer 14h is opaque. Incident light rays striking toned portions of the surface are intercepted and absorbed. Light rays striking untoned portions of the surface are reflected specularly or diifusely, or both, depending on the degree of glossiness or flatness of the layer 14h. The visual appearance of sheet 14, whether black, white or colored, matte or glossy, is the background appearance of the image.

Other substrate materials may be substituted for the acetate lm above described. Among such other materials are glass, polyethylene, cellophane, etc., or in fact any sheet material having acceptable physical characteristics coupled with a resistivity in the range above speciiied and the ability to accept a coating or photoconductive material.

ENCAPSULATED TONERS A particularly advantageous method of applying toner to the back of a substrate is by the use of encapsulation techniques. Methods are Well known in the art for enclosing particles or droplets of various materials in individual gelatinous coatings in sizes which may be as small as 5 microns or as large as 0.25 inch, as desired. Toners have been encapsulated as particles in the size range from 5 to 50 microns, triboelectrically charged by larger size carriers, and applied to charge images on top of the photoconductive layer to form a visible image. Cascade methods can be used to apply the toner to the charge image. The visible image may be fixed by bursting the particles with pressure. In these applications, the outer gelatinuous coating is triboelectrically charged by the 12 carrier, and the inner material is attracted to the charge image only because it is associated with the charged shell around it.

According to the present invention, regular liquid toners are encapsulated at full development strength, relying on the pigment particles, e.g. carbon black, to be charged by the regular dielectric dispersant and resin, rather than relying on the use of any outside carrier. Accordingly, the gelatinuous coating (pork skin gelatin, for example) merely serves to hold the regular toner concentration in individual capsules, and plays no part in the electrostatic transport of the toner to the appropriate parts of the image area. This being the case, the individual capsules can be dispersed in a binder and mixed with a solvent, to form a composition suitable for coating on a substrate.

In one embodiment of this invention, the back of a relatively insulating substrate is coated with such a dispersion (making certain that the dry resistivity of the coating is suiciently high to avoid shorting out the lines of flux and low enough to avoid interfering with charging).

After the sheet is charged and exposed, it is now necessary only to break the encapsulating shells of material around the toner particles (for example, iby passing the sheet between pressure rollers). This releases a uniform layer of liquid toner onto the rear of the substrate. If the pressure surface breaking the capsules is smooth and uniform, and the back of the substrate is inherently smooth and/ or lubricated, the released charged pigmented toner particles tend t0 move laterally and line up with the charge image electrostatic flux vectors, thereby forming a visible image. The gelatin shells constitute no more than about 15% by volume and merely spread evenly without materially affecting the resulting image.

A distinct advantage of this aspect of the invention is that all necessary materials for electrophotographic reproduction, the photoconductor, substrate and toner are combined in a single sheet that can be stored almost indefinitely. Thus, it eliminates the necessity of stocking toner, carrier, toner-replenisher, etc. and further eliminates the need of equipment for application of toner to the substrate layer after exposure.

Another advantage lies in the fact that the process is in effect a dry one, and can be made practically instantaneous, inasmuch as highly volatile solvents such as the freons can be used in place of the less volatile solvents heretofore required to provide a reasonable pot life.

Moreover, the use of a layer of encapsulated toner on the back of the substrate effectively contines the toner to that side of the sheet, and no special precautions are required in order to keep the toner olf the front of the sheet.

By coating either positive or negative toner on either transparent or opaque substrates, it is possible to get any type of copy desired, merely by changing the copy sheet.

yCoating an encapsulated toner on the back of a relatively insulating substrate also makes possible an appreciable increase in the effective photographic speed of this system. When the photoconductiye layer is exposed to light, the light causes loss of charge in the exposed areas. This discharge is not instantaneous, however, but requires a finite time interval. The degree of charge diterence between exposed and unexposed portions of the sheet is proportioned (within limits) to the duration of exposure, assuming constant intensity of light. If the sensitivity of the toner to small charge differences can be increased, then smaller charge differences, and hence a shorter exposure will suffice to produce a fully developed image.

Using a coating of encapsulated toner particles on the back of the substrate, the toner is in intimate contact with the sheet, and particularly so at the moment when the capsules are burst by pressure rollers or the like. The result of this intimate contact is to improve the sensitivity of the toner to the voltage differences constituting the latent charge image. Further increases in sensitivity may be achieved by applying a voltage across the pressure rolls to buck-out the large uniform voltage level across the sheet, thereby leaving only the actual image gradient to be developed. Even without the bucking voltage, the presence of a conductive roller surface behind the sheet acts to pull the field lines to the rear, thereby enhancing image density, particularly in large solid areas.

A cross-section of a sheet according to this embodiment is shown in FIG. 7, which shows a relatively insulating substrate having a photoconductive layer 11 on one face and a layer of encapsulated toner particles on the other face.

FIG. 8 depicts in somewhat diagrammatic form the manner in which the capsules are crushed after exposure to apply the toner to the back of the substrate. An exposed record sheet comprising substrate 10 coated on one side with a photoconductive layer 11, and on the other side with a layer of encapsulated toner particles 20, is passed between pressure rollers 21 and 22. One of the rollers, preferably the roller lwhich contacts the photoconductive layer, is coated with a thin insulating film 25, such as a ilm of Mylar polyester. An may be applied between the rollers by a suitable D.C. source such as generator 23. As the sheet passes between the rollers, the capsules are burst open, releasing liquid toner which collects on the areas of the sheet corresponding to the latent charge image, as indicated at 24. As the solvent portion of the liquid toner evaporates, the resulting image becomes permanent. The function of the insulating film is to prevent short-circuiting lbetween the rollers between sheets, or around the edges of sheets. The polarity of the applied.E.M.F. (if one is used) depends on Whether the image 1s positive or negative, and whether the voltage 1s intended to buck or enhance the voltage on the sheet. A squeegee or scraper blade 26 may be provided for the purpose of removing excess toner from roller 22.

A variation of the embodiment of the invention shown in FIG. 8 is shown in FIG. 9. In this embodiment, excess toner is removed from the back of the substrate by a release sheet 27. This sheet provides a means for removing toner from the clear areas of the image, instead of depending on the mobility of the toner to carry it from the clear to the dark areas of the image, or picking it up on the roller and subsequently scraping it oft.

As shown in FIG. 9, release sheet 27 is fed between rollers 21 and 22 in facing relationship to the encapsulated toner particles on the back of insulating substrate 10. As the capsules are burst by the pressure, the toner adheres to the appropriate portions of the substrate to form a visible image corresponding to the latent image. In the areas corresponding to the clear areas of the visible image, the toner is carried off :by the release sheet 27, as indicated at 28. The release sheet 27 is preferably somewhat tacky or bibulous, so that the toner will adhere preferentially to it, rather than to the relatively insulating substrate, in the clear portions of the image. It should not, of course, be so receptive to the toner that it twill pull toner away from the parts of the image area that are to be toned.

The toner adhering to the release sheets forms a negative of the visible image on the substrate, Iand may be used as a permanent record in its own right, instead of or in addition to the image developed on the insulating substrate. For example, if the release sheet is transparent, this process simultaneously provides an opaque positive and a transparent negative image, to or viceversa.

A further variation of the invention, similar to that shown in FIG. 9, is shown in FIG. 10. As shown in FIG. 10, the encapsulated toner is present as a coating 20 on the face of the release sheet 27a, rather than on the back of the relatively insulating substrate. Using this system, it is possible to make simultaneous positive and negative copies, as described in connection with the embodiment of FIG. 9. It is also possible to make multiple copies from a single latent image. To do this, the

photoconductive material is charged and exposed in the usual way. The back of the substrate is passed between rollers 21 and 22 in contact with the coated face of release sheet 27a, bursting the capsules and creating a positive image on the back of the substrate and a negative image on the face of the release sheet (or viceversa, as desired). The sheets are separated, another coated release sheet is placed in contact with the back of the substrate, and the assembly again passed through the rollers, for as many times as copies are needed. The number of copies so produced is limited only by the decay rate of the latent image on the photoconductive material. If necessary, the substrate may be wiped or squeegeed clean between copies, to prevent excessive build-up of toner.

SIMULTANEOUS EXPOSURE AND DEVELOYMENT In certain embodiments of the invention, it is advantageous to expose the photoconductive layer and develop a visible image on the back of the substrate at the samel time. This procedure is not possible with conventional electrostatic reproduction systems, -but is made possible according to this invention by virtue of the fact that exposure and development take place on opposite sides of the substrate, so that one does not of necessity interfere with the other.

One method of simultaneously exposing and developing is illustrated in FIG. 1l. As shown in FIG. ll, the substrate sheet 10, coated on one side with a photoconductive material 11, and on the other with encapsulated tone 20, is placed in contact with a transparent original such as a silver halide negative 29. The emulsion side of the silver halide negative is preferably placed toward the photoconductive layer 11,` for sharpest reproduction. A release sheet 27 is preferably placed on the side of the substrate next to the encapsulated toner 20. The sandwich is charged and then passed between pressure rollers 30 and 22. Roller 30 is hollow and transparent or translucent. A light 31 is disposed within the roller 30.

As the assembly passes between the rollers 22, 30, photoconductive layer 11 is exposed to the light through the photographic negative 29, and discharged in the clear areas of the negative. Simultaneously, the toner capsules are broken, and the toner is attracted to the appropriate areas on tre back of the substrate to form a visible image. Excess toner is carried off by the release sheet 27, which thereby acquires a visible image equivalent to that produced on the substrate, but with tone values reversed and laterally reversed to form a mirror image.

It is also within the purview of the invention to break the capsules just before the sheet is exposed so that during the exposure the toner is present as a thin liquid layer between the substrate and the release sheet.

If means are provided for keeping liquid toner in place against the substrate, it is possible to observe changing phenomena by way of the changing images formed by the toner. An embodiment of the invention which makes use of the technique is shown in FIG. l2. The device of FIG. l2 is an image wavelength converter cell for making visible an image composed of invisible radiation such as infra-red, ultra-violet or X-rays. The photoconductive layer 11 is selected to be responsive to the invisible radiation in question. Substrate v10 is of the same Igeneral nature :as those previously described and is charged. Cell 32 is enclosed by an insulating rear wall 33, a transparent front wall 34, and on the side walls 35 by any convenient insulating material such as rubber or plastic. Transparent front wall 34 is preferably conductive or provided with a transparent conductive iilm, and may be grounded, to assist in pulling the lines of ux through the cell.

Inside the cell 32 is a suspension 36 of colored pigment particles in a fluid dielectric dispersant.

The cell 32 is brought up against substrate 10 and held there snugly by any appropriate means (not shown), or

may be held manually. As invisible radiation constituting the image, represented by arrows 37, impinges on the charged photoconductive layer 1v1, 11 selectively discharges it and creates a latent image. The field lines of the latent image extend through the rear of the substrate, in the manner discussed above, and also extend through the insulating rear wall 33 of the cell. The suspended pigment particles arrange themselves as directed by the lines of flux, forming a visible image within the cell, which can be observed through transparent front wall 34. As the visible image impinging on the photoconductive layer changes, the visible image will change correspondingly, until the charge on the photoconductive layer has dissipated to the point where it is no longer responsive. If desired, the photoconductive layer may be recharged in situ, for example, by periodically subjecting it to a corona discharge. Otherwise, it must be periodically recharged in the usual way. Illumination for observation of the visible image is preferably an oblique illumination of rather moderate intensity, indicated by arrows 3'8.

FRONT TONING Although, as noted above, there are many advantages attaching to the application of toner to the rear of the substrate sheet rather than to the photoconductive face, nevertheless, this is not a limiting feature of all embodiments of the invention, and it is sometimes preferred for one reason or another to apply the toner to the front of the substrate, i.e. the face bearing the photoconductive layer.

Using encapsulated toners as described above, this application can be accomplished in a variety of ways. For example, the photoconductive layer may be charged and exposed in conventional manner, and then placed in faceto-face contact with a release sheet coated with encapsulated toner, and passed between pressure rollers in the same general manner as illustrated in FIG. 10, except that the record sheet is fed in with the photoconductive layer facing toward the release sheet instead of away from it. Also, in this manner of operation, the assembly should be fed in such a way that the insulated roller is in contact with the release sheet instead of the substrate-photoconductor sheet. The release sheet may again be tacky, and D.C. bucking or aiding voltages may be applied across the rolls, as previously described. The same positivenegative relationship now exists between the toner-carrying sheet and the surface of the photoconductive layer. Either or both may be used as the iinal record sheet.

In a variation of this process, the toner capsules may be coated directly on top of the photoconductive layer, subject to the limitation that the encapsulated toner layer must allow passage of sufcient light to form a latent image on the photoconductive layer. Alternately, the photoconductive layer may be exposed from the rear, for example, using as the substrate a plate or other transparent or translucent material. 'This embodiment of the invention is illustrated in FIG. 13.

As shown in FIG. 13, toner capsules are applied as a coating over photoconductive layer 11, which in turn is coated on a conductive coated glass substrate 10a. Exposure is made from the rear through the substrate using any appropriate light source, indicated at 31. Release sheet 27 is placed in contact with the layer of encapsulated toner either before or after exposure, as may be con- Venient under the circumstances, and the assembly subsequently passed between pressure rollers to burst the capsules and form the Visible image.

HALF-TONE REPRODUCTION Special problems arise when it is desired to reproduce half-tone material, i.e. material in which tones intermediate between pure white and solid black must be recognizably reproduced. Some degree of tone gradation can be obtained by virtue of the fact that the amount of toner attracted to a given point in the image area is proportional to the intensity of the latent image at that point. Control of the intensity of the intermediate tones by this method is very diicult, however, and the tendency is for toner to be attracted either in sufficient amount to render the image eld in the central portions ofthe black masses. attracted at all. Attempts to reproduce half-tones by conventional electrostatic reproduction methods therefore tend to produce extremely contrasty images, devoid or almost devoid of gray tones.

Even where half-tones are not involved, certain diculties arise when the material to be reproduced contains large 'masses of black (or of White, when the reproduction is to be a negative of the original). In such instances, the lines of flux are weakened .by the long path between central portions of the black areas and the nearest adjacent white areas, resulting in a weakening of the latent image field in the central portions of the black masses. The result of this weakening is that the central portions tend to print lightly, as compared with the peripheral portions of the massive areas, resulting in muddy or washed-out tones.

Both types of diiculty can be overcome by depositing the toner in the form of an array of closely-spaced particulate dots on the surface of the photoconductive layer, as illustrated in FIG. 14. The sheet as illustrated in FIG. 14 comprises a substrate 10, a photoconductive layer 11 and a coating of encapsulated toner 20 deposited over the photoconductive layer in the form of discrete dots 39, separated by open spaces. When the layer 11 according to this embodiment is charged and then exposed to actinic radiation impinging on the sheet from the front, or toner-coated side, the dots of toner mask the photoconductive layer 11, and confine the discharging action of the radiation to the open spaces between the dots. Thus the pattern of toner blocks light in the areas covered 'by toner and lets it through in the clear areas between. This breaks up the large solid areas producing many individual vertical lield .vectors in Ethe solid area of the latent charge image, `giving good half-tones as well as line and letter development. Because of the manner in which the field is broken up, every discharged point is closely adjacent a point which remains charged because of the masking effect of the toner dot pattern, so that lengthening of the flux lines and consequent weakening of the field is avoided, even in the central portions of large dark areas.

This technique is not limited to front development, but may be used also for toning on the rear of the sheet as well, particularly when the exposure is also made through the rear of the sheet.

DIRECT ELECTROSTATIC RECORDING In certain aspects of the present invention, the information to be recorded is an electrostatic pattern to begin with, and there is no need to employ a photoconductive layer in order to convert a pattern of actinic radiation into an electrostatic charge pattern.

One embodiment of such an aspect of the invention is illustrated in FIG. 15. Certain types of electrostatic recording place an electrostatic charge on an insulating layer having a conducting backing via discharge from metal pins either singly or in banks and rows, which may be energized by separate pulse circuits or on the face of a cathode ray tube by means of an electron beam. In the conventional practice, the charge image produced on the sheet has been developed by conventional methods, using liquid or dry toners on the insulating side of the sheet. If, instead of a conductive backing, the sheet is provided with a relatively insulating backing, the charge image will still form, but the electrostatic flux line will not be shorted out by the conductive backing, and the image can be developed from the rear.

In the embodiment of the invention shown in FIG. 15, the charge image is provided by electrostatic discharges from an array of pins 40. The charge image is received upon a composite sheet comprising a dielectric, highly insulating film 41, a relatively insulating support base and a coating of encapsulated toner 20 on the rear of the support base. As the charge image is impressed on the composite sheet by the pins 40, or shortly before or after, depending on the construction of the apparatus, the sheet is pressed by grounded roller 42 against the pins, or against any suitable back-up device such as another roller (not shown) with sufficient force to burst the toner capsules. The toner adheres to the rear of the sheet in the charged areas to form indicia 43, corresponding to the pulse pattern received from the pins. Excess toner is carried off by the roller and is removed therefrom by scraper blade 26.

In this, as in the embodiments previously described, a tacky release sheet may be employed to remove excess toner, and bucking or aiding voltages may be applied as desired by way of the pressure roller.

This embodiment of the invention is particularly useful for applications requiring high speed and/or rapid accessibility, such as teletype, facsimiles, computer print-out, address labeling and the like.

MAP MAKING Various kinds of maps, particularly military maps, nautical charts and the like, are commonly made at present by multi-color lithographic methods. For some time, attempts have been underway to develop an electrostatic method of printing multi-color maps, by way of providing a number of color-separation negatives corresponding to the number of colors to be printed. A conventional type of zinc-oxide coated paper is charged by corona discharge, exposed to one of the negatives, toned with toner of the corresponding color, dried, and recharged for exposure to the next color-separation negative. In this way, as many as five separate negatives may be printed, one atop the other, on a single sheet of paper.

This method shows considerable promise, but is subject to certain disadvantages. For one thing, there iS a tendency for map information to be lost in the event the zinc oxide coating chalks off under rugged field use. Also, a serious disadvantage of the process is that, after a portion of the paper has been toned in response to one of the separation negatives, the zinc oxide coating in the toned region is no longer photoconductive. Thus, where two lines of different colors cross, for example, a road crossing a land contour line, the first line deactivates the zinc oxide so that it can no longer be charged and discharged by light. Also, if the first toner is a good insulator, it will simply hold the next charge and attract the next and all subsequent toners to itself.

Some methods of overcoming these difficulties have been proposed. It has been suggested, for example, to employ photoconductive toners which could be charged, exposed and toned in the same manner as the original photoconductive layer. It has also been proposed to employ special binder resins for the zinc oxide, such that the binder is softenable by the toner and dispersant, thereby allowing the toner to settle into the zinc oxide coating and allowing some zinc oxide particles to be exposed above the toner so that it can again be charged and 1ightdischarged.

These methods have met with limited success. Both require special materials. Also, the latter method permits the Zinc oxide binder resin to migrate into the toner dispersants. This contamination of the dispersant eventually causes the toner mix to lose its triboelectric charging ability.

According to the present invention, the charging and exposure steps are similar to those described above, but the toner is applied to the rear of the sheet. When this is done, the zinc oxide layer retains its ability to be charged and subsequently discharged by light, and where features are to be superimposed on the map, such as crossing lines, for example, one toner simply prints right over its predecessor.

Application of toner to the rear of the sheet may be carried out in a variety of ways, as suggested earlier herein, such as Kiss rollers, wicks, gelatin pods, etc. It may also be applied by using a separate release sheet coated with the appropriate colored toner in encapsulated form, after each exposure, after the fashion illustrated in FIG. 10.

`MICROF ILM BLOW-BACK A typical microfilm reader-printer, modified according to an embodiment of this invention, is illustrated diagrammatically in FIG. 16. In such systems, it is usual to provide an optical system whic includes a mirror. The lens system is such that it produces a reversed image of the microfilm copy. When the reversed image is reflected from the mirror to the ground glass for viewing, it appears there is a right-reading image.

As shown in FIG. 16, the apparatus comprises a reector 44, a light source 31, a condensing lens 45, a projection lens 46, a mirror 47, and ground-glass screen 48. In conventional reader-viewers the mirror is swingable to an alternative position at 47a, and a suitable support 49 is provided on which a sheet of record material 50 may be placed. In the apparatus according to the invention, these features may be dispensed with, and the mirror may be and preferably is fixed in the position shown in solid outline at 47. The reason for providing lthese features in conventional machines was to provide means for removing the mirror from the optical train, so as to compensate for the reversal of the image produced by the record medium used (commonly either an electrolytic or a stabilization silver halide system).

According to ythis invention, the record medium is a sheet having a charged photoconductive layer on a relatively insulating substrate. The sheet is placed against the ground-glass screen with the photoconductive side facing the screen, and exposed to the mirror-reflected image of the microfilm frame 51. A removable clear glass plate 52 is preferably provided, which is normally in the position shown in solid outline, but can be temporarily moved to the position shown in dotted outline at 52a. When the record sheet is being inserted, the clear glass plate 52 is swung out, and then returned to its normal position to hold the record sheet against screen 48. After exposure, the sheet is removed from the screen and toned on the rear. If it were toned on the front, as in conventional electrostatic reproduction methods, the developed image would be a mirror image. When the sheet is exposed on the front and toned on the rear, however, a right-reading image is obtained.

The embodiment of the invention just described is advantageous over conventional microfilm blow-back processes in a variety of ways. It eliminates the need for a movable mirror, which eliminates one possible source of errors. If the mirror, when in reading position is not seated precisely in its designed position, the optical path from the projection lens to the ground glass will be slightly shorter or longer than that from the projection lens to the easel, and when the system is sharply focused on the easel for reading, it will be out of focus when the mirror is swung down for reproduction.

Also, the type of reproduction desired is usually an opaque positive, and the microfilm is usually a transparent negative, `and conventional systems are so designed as to produce an opaque positive from a transparent negative. The embodiment of the invention just described is perfectly suited to perform this type of reproduction.

However, other types of reproduction are sometimes desired. The blow-back may be desired to be made on vellum or translucent stock for subsequent diazo reproduction or the like. Also, the micro-storage may be positive transparent film, or on a positive opaque microcard. The reproduction processes commonly used in commercial microfilm reader-printers are basically reversal processes, and there are not easily converted to positive toned with a positive toner. If it is desired to make a positive copy from a positive original, it is necessary merely to substitute a negative toner for the positive toner.

If viewing of the copied material is not required, it is of course possible to use rear-of-sheet toning in a continuous roll-to-roll or step frame microlm print-out without viewing as is presently done in some commercial systems using a selenium coated drum.

CATHODE RAY TUBE PRINT-OUT FIG. I17 illustrates another embodiment of the invention, which provides a hard-copy record of cathode ray tube displays. Cathode ray tube 53 is a tube of the type having a fiber-optic display face 54. In front of the display face is a glass plate 55. The space between the display face and plate 55 forms a narrow channel for passage of record paper in front of the tube. Preferably, means (not shown) are provided for moving plate S in- Wardly at appropriate times, to press the paper against display face 54.

The record paper is a relatively insulating substrate coated on one side with a photoconductive material and preferably coated on the other side with encapsulated toner.

The tube is used for normal Viewing until such time as a permanent record is desired, at which time a cycle is initiated which proceeds as follows:

(l) The normal electron beam scan is turned off, allowing the phosphors on the fiber-optic surface inside the tube to become dark.

(2) Feed rollers 56, powered by any appropriate means, such as an electric motor (not sho-wn) transports a sheet of record paper from a supply 57 thereof, through guides 58 and corona discharge device 59, and into the space between glass plate 55 and the outer face of the fiber-optic display element 54. The photoconductive surface faces the display face 54.

(3) The glass plate 55 may be moved forward by any appropriate means (not shown) to assure good contact between the photoconductive surface ll and the fiber ends, for better resolution.

(4) The electron beam is again turned on for a few scans, long enough to discharge the photoconductor and form a latent image but as short as possible, to avoid image smear, and again turned oif.

(5) The sheet is advanced and removed from the apparatus by activation of rollers 60. If the sheets used are coated with encapsulated toner, these rolls 60 break the capsules causing development of the image and providing a finished hard copy. Otherwise, any conventional toning method may be used.

(6) As soon as the trailing edge of the sheet passes through the exit rollers 60, the scan may be turned on again and normal viewing resumed.

The entire cycle just described may be completed in a matter of two to three seconds, which is short enough to assure reproduction of most subjects without their changing significantly. Also, for data print-out applications, the cycle can be triggered by the start of transmission, to assure that the sheet is imaged while transmission is taking place.

A variation of this device, where viewing is not required, would employ a continuous tlow of record paper past a cathode ray tube with a line array of fibers. (Such a tube is supplied by the General Dynamics Corp).

TONER O'N REAR OF RELEASE SHEET In the embodiments of the invention so far described, the toner, whether encapsulated or otherwise, has been placed in contact with the substrate sheet carrying the photoconductive layer. In some of the embodiments, this has been done by applying a coating of encapsulated toner to either the front or the back of the photoconductor-substrate sheet itself, in others by contacting front or back of the photoconductor-substrate sheet with a release sheet having a coating of encapsulated toner on the side of the release sheet facing the photoconductorsubstrate sheet, and in still others by conventional direct application of toner in liquid or powder form.

In another aspect, this invention contemplates a method in which the toner never cornes in contac-t with the photoconductor-substrate sheet. Instead, the toner is released on a separate sheet, while the separate sheet is in contact with the photoconductor or with the back of the substrate.

This method, in one embodiment is illustrated in FIG. l\8. As shown in FIG. 18, a light image is projected through lens 61, as indicated by the arrow, onto a photoconductive layer 11, carried o-n a relatively insulating substrate 10. Closely behind substrate '10i is another relatively insulating `sheet y62, coated with a layer of encapsulated toner 2,0. It is to be noted that in this embodiment of the invention, the toner capsules are on the face of sheet 62 which is furthest away from the substrate.

Between substrate lil and sheet 62 there is a small air gap. This gap should be made as small as possible. In actual operation, it is practically non-existent because the electrostatic attraction of the charged photoconductor-substrate sheet attracts sheet 62 and holds it in intimate contact with the substrate.

The light image discharges the photoconductor in a pattern constituting a latent charge image, and the charge image is manifested as an electrostatic force iield both in front of and behind the substrate sheet 10, as previously discussed. The force eld is symbolized in FIG. 18 by arrow 13, representing a line of electrostatic Jinx. The force eld extends through sheet 62, as indicated. The capsules of toner on the rear of the sheet 62 are broken in any convenient manner, while sheet 62 is still in contact with substrate |10, and the toner forms a visible image on the back of sheet 62, in substantially the same way as, in previously discussed embodiments of the invention, it forms a visible image on the back of the substrate I10 itself. If substrate 10 is suficiently strong and rigid, the capsules can be broken by passing a pressure roller over the surface of the capsules, without moving the assembly from the position shown in FIG. 18. Otherwise, the assembly comprising the photoconductor-substrate sheet and sheet 62 can be passed as a whole through a pair of pressure rollers. Instead of using encapsulated toner, the back of sheet 62 can be toned in any of the conventional rlnethods, while it is maintained in contact with substrate This method of producing a visible image has a number of distinct advantages. The photoconductor and its substrate can be kept permanently in the machine and used many times over. This greatly reduces the per-copy cost of reproduced material, because it does not involve a consumption of photoconductive material for each copy made.

This method can be usefully employed in a camera, using a moving double corona charger, a slit focal plane shutter and a pressure roller, all moving in the order named past the recording sandwich, sequentially charging and exposing the photoconductive material and developing the copy sheet. These operations, carried out in rapid sequence, permit the use of short dark decay photoconductors of useful camera speed, assuring that all areas 2l of the sheet will have equal charge, exposure, dark decay and development time.

A variation of the process just described is to place the copy sheet 62 not in contact with the back of the substrate, but on top of the photoconductive layer, with the side of the copy sheet to be toned facing away from the photoconductor. The image will then conform to the field lines emanating from the surface of the photoconductor, and will give the same image as one toned directly on the photoconductive layer, for the reasons set forth in the section entitled Theoretical Discussion. -In this case, however, it is necessary to use a mirror or equivalent reversing means to obtain a right-reading image, unless copy sheet 62 is transparent and designed to ybe viewed from the side opposite that to which the toner is applied.

This method may also be used to produce multiple copies simultaneously, by stacking a plurality of insulating copy sheets behind the substrate, each of the copy sheets having its own coating of toner capsules. A back conductive electrode may be used to extendl the field lines. Such a back electrode may have a boosting DC. voltage, and/ or may serve as a pressure roller or pressure back-up plate to assist in bursting the toner capsules. This would have particular use in non-impact print-out from computers and the like, where several copies are sometimes required.

LITHOGRAPHIC MASTERS It has already been proposed to use electrostatic reproductions, for example, on zine oxide in a resinous binder or on an organic photoconductor, as lithographie masters. Lithographie reproduction methods depend upon differential wetabilities, the background being a hydrophilic suface which is kept moistened with water and hence repels the greasy ink, while the areas to be printed are rendered oleophilic in one way or another, and therefore accept the ink.

When a conventional electrostatic reproduction is to be used as a lithographic master, certain difiiculties arise. The binder or resin used for photoconductive material must present both a good hydrophilic surface and a good photoconductive one. For cost and other reasons, these requirements are frequently conflicting.

IWhen toner is applied to the rear of the sheet according to the present invention, however, these two functions are completely separated, and the insulating substrate may serve only in the single capacity of a good hydrophilic surface. If desired, the rear surface may be modified by chemical treatment or the like, or by applying a separate rear coating to improve the offset plate characteristics. None of these expedients effect the photoconductive layer, and the only precaution necessary is that any treatment or coating should not materially reduce the insulating properties of the substrate layer, nor provide a conductive layer to short out the lines of flux at the rear of the substrate sheet.

Using a hydrophilic substrate toned on the rear with an oleophilic toner, the developed sheet can be used as an offset master for either direct lithography (which requires that the image on the master be a mirror image of the desired print) or the more conventional offset lithography, wherein the ink image is transferred from the master to an offset blanket and thence to the final copy sheet.

DIAZO MASTERS The present invention lends itself admirably to the production of Diazo masters for subsequent diazo reproduction processes. Since a correct left right image can be formed on the rear face of the substrate, the diazo sensitive surface of the diazo paper can be placed in direct contact with the rear face image. The result is better resolution, contrast and edge definition. The direct contact of the master and the diazo paper prevents light form passing between the edges of the indicia on the substrate and the diazo paper. It will be apparent that when used for this purpose, both the photoconductor and substrate must be good transmitters for the actinic ultra-violet rays required in the diazo process.

HOMOGENEOUS PHOTOCONDUCTIVE SHEETS Instead of coating the photoconductive layer onto the surface of a relatively insulating substrate, the photoconductive material (zinc oxide, or the like) may be blended into the insulating material such as cellulose acetate prior to forming the sheet. The resulting mixture is then rolled or cast into a sheet or film. The so-formed sheet or film is not homogeneous in the strict sense of the word, but may be so described because it is uniformly photoconductive throughout its thickness, instead of Ibeing a two-layer composite of a photoconductive layer and an insulating substrate layer. Sensitizing dyes and the like may also be incorporated into the composition as required. For most purposes, the sheet should be made in a thickness of 0.002 to `0.004 inch, which gives good charge retention and provides a high contrast in the latent charge image and in the eventual visible toner image. This thickness is also ideal for sheet handling.

Using a homogeneous sheet of this type, the electrostatic field lines of the charge image need not penetrate even a thin relatively insulating substrate in order to extend to the rear of the sheet. Because of this fact, resolution and solid area fill-in are improved. Also, charging is made easier by the fact that the resistance through the sheet is lower than it would be when using a two-layer type of sheet.

The homogeneous sheet according to this embodiment of the invention possesses a number of other significant advantages over conventional types of photoconductive sheet materials. It is not subject to the possibility of aking inherent in a sheet coated with a solid powder material in a binder resin. It also shows less tendency to curl because of differences in thermal expansion, moisture absorption, or the like. Moreover, the sheet is of a uniform white appearance, and is more acceptable to many users because its appearance and hand approach those of paper.

Where cellulose acetate is used as the binder it further protects the photoconductive material from the effects of moisture and the like.

APPARATUS A number of types of apparatus have been described above in greater or less detail, in connection With the description of various materials and methods according to the invention. Numerous other types of apparatus are contemplated by the present invention and some of these are suggested below. For the sake of brevity they are not described in detail, but they are considered to be adequately disclosed to those skilled in the art by the following brief descriptions taken together with the foregoing extended disclosure of the principles underlying the invention:

(l) A microfilm camera using a transparent photoconductive material coated on a transparent relatively insulating substrate (see FIG. 5) in which lthe film is charged, exposed to the original through a normal optical reduction system, developed with toner on the rear of the sheet and fixed.

(2) A still camera for recording indoor and outdoor scenes, portraits, documents, etc., in which the film is charged, exposed through a normal camera lens on film in, for example, a 35 mm. or 4" x 5" for mat or the like, developed on the rear of the sheet and fixed.

(3) A motion picture camera in which the coated film is cut to conveniently sized strips such as 8, 16, 35 or 70 mm. strips with or without sprocket holes, and provided as desired with a magnetic sound track, or an optical sound track developed on the same photoconductive surface as used for the optical record.

(4) A reflex contact document copier provided with means for charging and exposing a record sheet having a photoconductive layer on an insulating substrate, and subsequently toning the rear of the substrate sheet.

A camera type device in which the photoconductive layer is reused many times, and deposits an image, which can be positive or negative as desired, on a release sheet which may be opaque or translucent, and the image is iixed on the release sheet. If the release sheet is opaque and to be viewed by reflection, an odd number of reilecting surfaces or the equivalent should be provided in the optical system, in order to provide a correct left-right image.

(6) A micro-storage device in which a smooth photoconductive record sheet is exposed, toned, and rolled up, with a protective interleaf if desired. The sheet can be unwound and viewed in a microfilm reader and, if necessary, the existing image can be wiped 01T and the frame in question recharged, reexposed and retoned to form a new image, without disturbing the images on the remainder of the roll.

Many other types of apparatus for making use of various embodiments of the present invention will, of course, readily occur to those skilled in the art, in the light of the foregoing disclosures.

The present invention provides improved methods, appara-tus and recording media for electrostatic reproduction. It provides electrostatic reproduction methods and materials which avoid the necessity of producing a permanent toned image on the top of a photoconductive layer, and oiers the potentiality of producing right-reading copy by retlex contact printing from an opaque original. Electrostatic reproduction systems according to the present invention are practically independent of ambient humidity condi-tions.

The present invention also provides electrostatic reproduction systems that eliminate the necessity for handling toners, replenishers, carriers and the like in bulk form. It also provides systems which do not involve the use of short-pot-life toner compositions.

This invention moreover provides electrostatic reproduction systems suitable for the reproduction of half-tones and of subjects requiring rapid access such as aerial photographs, computer print-out, teletype and the like.

While this invention has been described with reference to certain preferred embodiments and illustrated by way of certain drawings and examples, these are illustrative only, as many alternatives and equivalents will readily occur to those skilled in the art, without departing from the spirit and proper scope of the invention. The invention is therefore not to be construed as limited except as set forth in the appended claims.

Having thus fully described the invention, what is claimed as new and desired to be secured by Letters Patents of the United States is:

1. A method of reproducing indicia comprising the steps of charging the photoonductive face of a sheet-like member comprising an insulating substrate and a coating of a photoconductive material on one face of said substrate, said substrate having a resistivity not less than l 1010 ohm-cms. and not more than two orders of magnitude greater than that of the photoconductive coating, directing light from the indicia to be reproduced upon the charged photoconductive coating to produce a flux image upon the uncoated face of the substrate and applying an electrostatically sensitive toner to the said uncoated substrate face to develop a visible image from the flux image.

2. The method according to claim 1 in which the toner is in the form of encapsulated particles and is applied to the substrate face by crushing the encapsulated particles.

3. The method according to claim 1 in which the toner is in the form of encapsulated particles secured to the substrate face furthest from the photoconductive material and is applied to the substrate face by crushing the encapsulated particles.

4. The method according to claim 3 in which the toner is in the form of encapsulated particles secured to a release sheet and is applied to the uncoated substrate face by crushing the encapsulated particles between the substrate and the release sheet, and thereafter removing the release sheet from the substrate.

5. The method according to claim 4 which the toner is in the form of encapsulated particles secured to a release sheet and is applied to the uncoated substrate face by crushing the encapsulated particles between the substrate and the release sheet, thereafter removing the release sheet from the substrate, and applying successive release sheets having toner theeron to produce a plurality of visual images on the release sheets from the iiux image on the substrate.

6. A method of reproducing indicia comprising the steps of charging the photoconductive face of a sheet-like member comprising an insulating substrate and a coating of a photoconductive material on one face of said substrate, said substrate having a resistivity not less than l 1010 ohm-cms. and not more than two orders of magnitude greater than that of the photoconductive coating, directing light from the indicia to be reproduced upon the charged photoconductive coating to produce a flux image upon the uncoated face of said su'bstrate furthest from the photoconductive material and the photoconductive face and applying an electrostatically sensitive toner to the said substrate face and the photoconductive face to develop a visible image from the llux image on both faces of the sheet-like member.

7. The method according to claim 1 in -which the toner is transferred to a separate copy sheet.

8. A method of reproducing multi-colored indicia by successive application of light from a plurality of different colored images to a record surface, comprising in combination the steps of providing a record medium in the form of an insulating substrate and a coating 0f photoconductive material on said substrate said substrate having a resistivity of not less than 1 1010 ohm-cms. andl not more than two orders of magnitude greater than that of the photoconductive coating, charging the photoconductive material exposing said photoconductive coating to an actinic radiation image corresponding to the first of said colored images, thereby creating on the uncoated side of the substrate a flux image corresponding to said rst radiation image, toning said substrate on the side opposite said photoconductive layer with a rst colored toner and repeating said, charging exposure and said toning for each additional different colored image to be reproduced on said record medium.

9. A method of generating indicia from electrostatic discharges from metal pins or the like, comprising in cornbination the Steps of providing a record sheet in the form of a highly insulating dielectric layer laminated to an insulating layer having a resistivity of not less than l 101o ohm-cms. and not more than two orders of magnitude greater than the highly insulating dielectric layer, applying an electrical discharge to said dielectric layer, thereby creating an electrostatic ux image on the face of the record sheet opposite the dielectric layer, and developing a visible image corresponding to said ux image by applying toner to the surface of said insulating layer opposite the highly insulating dielectric layer.

10. A method of preparing a lithographie master, comprising in combination the steps of providing a master medium in the form of an insulating hydrophilic substrate and a coating of photoconductive material on said substrate said insulating substrate having a resistivity of not less than l 1010 ohm-cms. and not more than two orders of magnitude greater than that of the photoconductive material, exposing said master medium to an actinic radiation image of the indicia to -be reproduced, thereby creating on the uncoated side of said master medium a flux image corresponding to said indicia, and developing a printing image by toning said hydrophilic substrate on the face opposite the face bearing said coating of photoconductive material with an oleophilic toner.

ll. A recording medium useful for electrostatic reproduction comprising in combination an insulating substrate and a coating of a photoconductive material on one face of said substrate, said substrate having a resistivity not less than 1 1010 ohm-cms. and not more than two orders of magnitude greater than that of the photoconductive coating, and further including a coating of encapsulated toner material on the surface of said substrate opposite said photoconductive coating.

12. The method of producing indicia according to claim 1 in which the toner is fused to the sheet-like member to form a permanent image.

13. The method according to claim 1 in which the toner is carried by a second insulative sheet which is brought into contact with the first insulative sheet with the toner on the side furthest removed from said rst insulative sheet.

References Cited UNITED STATES PATENTS 2,955,938 10/1960 Steinhelper 96-1 2,965,481 1'2/ 1960 Gandloch 96-1 5 2,987,395 6/1961 Jarvis 96-1 3,080,318 3/1963 Claus 252-621 3,084,061 4/1963 Hall 117-17.5 3,429,701 2/ 1969 Koehler 96-1 10 GEORGE F. LESMES, Primary Examiner I. C. COOPER III, Assistant Examiner U.S. C1. X.R. 

